The invention relates to improvements in methods of and in apparatus for ascertaining certain characteristics of streams of fibrous material, such as tobacco or filter material for tobacco smoke. More particularly, the invention relates to improvements in methods of and in apparatus for ascertaining the density of a continuous stream of tobacco or other fibrous material while the stream is in the process of being transported by a foraminous conveyor.
It is well known to ascertain the density of a stream of fibrous material by causing the stream to advance across a beam of radiation issuing from a suitable source and impinging upon a detector which serves to generate signals denoting the intensity and/or other characteristics of that portion of the beam which has penetrated all the way through the stream. Such signals can be processed into so-called density signals, i.e., into signals which denote the density of successive increments of the stream advancing between the radiation source and the detector.
Cigarettes, cigarillos, cigars, filter rod sections and other rod-shaped articles of the tobacco processing industry must be tested for a number of reasons, particularly to ascertain their appearance, taste, weight, rate of flow of tobacco smoke toward the mouth of the smoker and/or other characteristics. This is important because the makers of rod-shaped smokers' products (hereinafter called cigarettes for short) expect that the afore-enumerated as well as certain other desirable characteristics of the cigarettes remain unchanged for any selected interval of time. This ensures that the smoker can recognize her or his brand, i.e., that the smoker can recognize the preferred brand by taste, feel, rate of flow of tobacco smoke and other characteristics. One of the presently preferred and important procedures to rapidly detect and eliminate changes in the characteristics of various brands is to continuously monitor the density of the rod which is to be sub-divided into discrete cigarettes and/or to monitor the density of discrete cigarettes. The results of density measurement are utilized to influence one or more important steps during making of the cigarettes.
One of the presently preferred procedures which are relied upon to ascertain the density of cigarettes is to use a measuring device which is equipped with a source of corpuscular radiation (such as beta rays) and with an ionization chamber which performs the function of a transducer by generating signals denoting the intensity of radiation which has been emitted by the source and has penetrated all the way through successive increments of a continuous tobacco stream. Reference may be had, for example, to commonly owned U.S. Pat. No. 4,889,139 granted Dec. 26, 1989 to Heitmann.
Published British patent application No. 2 179 444 discloses a different measuring device wherein the radiation source emits optical light. Such light is caused to penetrate through an unwrapped moving stream of fibrous material, and the radiation which has penetrated through and beyond the stream is monitored by an optoelectronic transducer serving to generate appropriate signals which can be processed into signals denoting the density of the respective increments of the moving stream. The arrangement is such that the beam of optical radiation traverses a stream of fibrous material which advances between two spaced-apart channel walls while the stream is being compacted in a direction which is substantially parallel to the two walls and is normal to the direction of advancement of the stream with a foraminous conveyor. The densifying action is furnished by subatmospheric pressure in a suction chamber which is adjacent the stream-advancing reach of the endless foraminous conveyor.
A drawback of the just described density measuring method and apparatus is that the results of density measurement are overly influenced by fluctuations of density of the stream between the source of optical radiation and the optoelectronic transducer. The reason is that the radiation is caused to pass through the stream in a direction at right angles to the direction of compacting action of subatmospheric pressure in the suction chamber.